In optical wavelength division multiplex (WDM) passive optical networks (PONs) a plurality of optical network units (ONUs) are connected to a central node, also referred to as optical line terminal (OLT). PONs enable a bi-directional point-to-point (PtP) connection between each ONU and the OLT using dedicated optical channels, i.e. for each PtP connection a pair of downstream and upstream signals having predetermined optical wavelengths is used. In general, the downstream channel signal transmitted from the OLT to the respective ONU and the upstream channel signal transmitted in the reverse direction may have identical or different optical wavelengths. The plurality of downstream and upstream optical channel signals is transmitted as a respective combined downstream and upstream WDM signal within a WDM transmission link.
It is rather difficult to switch on a tunable optical transmitter, such as a tunable laser (in the following, the term “tunable laser” is equivalently used for any type of tunable optical transmitter), within an optical transmission system, such as a WDM system, wherein the optical transmission system does not allow the optical transmitter to start a sweep at an arbitrary wavelength. In such a WDM system, a plurality of optical wavelengths or optical channel signals in a given downstream or upstream transmission direction is combined via a coupler, a splitter or a combiner to an optical WDM signal. A wavelength insensitive coupler does not show any band filter properties with regard to each of the respective optical channel signals and thus the possibility exists that a channel signal of a laser that is switched on disturbs other channel signals or interferes with those, provided that this laser is not correctly tuned with regard to its wavelength. That is, if a newly added laser or a laser that has been switched off is switched on, and the laser wavelength (i.e. the center wavelength of the respective optical channel signal) does not lie with sufficient accuracy within the channel bandwidth that is dedicated to the respective optical channel signal, the respective optical channel signal may at least partly lie within the channel bandwidth of a neighboring optical channel. The same applies if during a tuning process of a tunable laser the laser wavelength is varied, e.g. swept over a predetermined interval, to such an extent that at least part of the optical spectrum of the optical channel signal lies within a neighboring optical channel bandwidth.
Such wavelength-independent couplers may especially be used in order to combine the optical channel signals created by a plurality of ONUs with a respective upstream optical WDM signal and to split the downstream optical WDM signal with respect to its optical power so that each ONU receives the full downstream WDM signal. Such a wavelength independent coupler may be part of a passive optical network defining PtP transmission links between the OLT and the ONUs. Of course, each ONU is configured to extract the desired optical channel signal from the downstream WDM signal, which is dedicated to the given PtP transmission link.
In many mass applications, low-cost tunable lasers will be required in the near future, especially at the consumer side of an optical WDM transmission system, i.e. within the ONUs. Owned dedicated wavelockers (WL) do not make such lasers any cheaper and an individual pre-calibration is expensive. However, as explained above, using, at one end of a WDM transmission link (especially the side of the ONUs of a PON), such low-cost tunable lasers, which cannot guarantee that the transmission wavelengths lie within the dedicated channel bandwidths with sufficient accuracy, in connection with a simple optical splitter for combining the plurality of optical channel signals to the respective (upstream) optical WDM signal causes the problem, that the optical signal created by such a low-cost laser may interfere with other channel signals already in use, if the optical signal does not lie within the dedicated optical channel bandwidth.
At the OLT, an arrayed waveguide grating (AWG) may be used for demultiplexing the received upstream WDM signal and for multiplexing the optical channel signals created by the OLT optical transmitters. Thus, the optical channel signal created by a tunable optical transmitter in the OLT and supplied to a dedicated channel port of the AWG is integrated into the downstream WDM signal, if the wavelength of the optical channel signal matches the dedicated channel bandwidth with sufficient accuracy, only. However, if a cyclic arrayed waveguide grating (CAWG) is used, for example to enable the use of different optical bands for the downstream and upstream optical channel signals, not only a downstream optical channel signal at the dedicated downstream channel bandwidth that is supplied to the respective channel port of the CAWG is output at the WDM port of the CAWG, but also an optical channel signal created by the same optical transmitter having a corresponding wavelength that lies in another order of the CAWG. If, in this case, the downstream optical WDM signal is combined with other downstream optical channel signals or WDM signals using a dedicated optical bandwidth, which overlaps with or comprise the bandwidth of the respective “other” order of the CAWG, interference between one of these channel signals and the channel signal of the laser to be tuned may occur, too, if the wavelength of the laser to be tuned is sufficiently far off the respective target wavelength, i.e. the center of the dedicated channel bandwidth.
The problems described above occur, for example, when a next generation PON 2 system (NG-PON2-system) is used, which has been defined by the responsible ITU committee (Recommendation ITU-T G.989.1), or when an NG-PON2 system is integrated in a more complex environment integrating an NG-PON2 system in an existing PON. An NG-PON2 system may be realized as a time and wavelength division (TWDM) PON part or a PtP WDM PON or as a combination thereof. The problems described above especially occur with transmission systems that do not work in burst mode, like TWDM systems do, but with WDM transmission systems that use a dedicated optical wavelength for continuously transmitting an optical signal created by an optical transmitter arranged at one end of the transmission link to an optical receiver arranged at the other end of the transmission link.